In a line type modulation system as a mainstream of the third generation mobile telecommunications (3G) and the prospective wireless communications, it is desired to realize a transmission apparatus capable of amplifying with no distortion at high power efficiency. The line type modulation involves fluctuations in envelope, and therefore an amplifier is designed and operated by taking a ratio (Peak to Average Ratio) between a signal peak and average power into consideration. Hence, such a problem exists that the transmission apparatus decreases in power efficiency.
A transmission circuit apparatus as depicted in FIG. 1 is exemplified as one of the prior arts (refer to, e.g., Patent document 1). According to a technology of this transmission circuit apparatus, as illustrated in FIG. 1, a data generator 5 outputs angle modulation data defined as angle component of a modulated wave and amplitude modulation data defined as an amplitude component of the modulated wave, respectively. Herein, a carrier wave c(t) having a frequency fc may be expressed in the following formula 1.c(t)=cos(2πfct)=Re[ej2πfct]  (Formula 1)
A modulated wave e(t) with respect to the carrier wave c(t) may be expressed in the following formulae 2 and 3 by using a complex envelop E(t).e(t)=A(t){cos(φ(t))·cos(2πfct)−sin(φ(t))·sin(2πfπct))=Re[E(t)ej2πfct]  (Formula 2)E(t)=A(t)·ejφ(t)  (Formula 3)
Wherein A(t) represents amplitude (amplitude component) of the modulated wave. The symbol φ(t) denotes a phase (angle component), the frequency is defined as a differential of the phase, and hence both of the phase and the frequency may be generally treated as the angle components.
A frequency modulator 1 depicted in FIG. 1 generates a signal that is modulated with an angle component φ(t). On the other hand, the amplitude component A(t) is converted by a Σ−Δ modulator (or Δ−Σ modulator) 3 into a pulse width modulation (PWM) signal. The amplitude component converted into the PWM format, i.e., a PWM signal A′ (t) and the signal undergoing the angle modulation are modulated by the amplitude modulator 2. An output of the amplitude modulator 2 becomes a signal into which the angle modulated wave having fixed amplitude is set ON or OFF depending on “1” or “0” of the PWM signal.
The amplitude of the amplitude modulator 2 such as this takes a value “0” or a fixed value. Accordingly, even when the output signal is amplified by an unillustrated class B or C power amplifier disposed at a rear stage of the amplitude modulator 2, no distortion occurs, and besides the signal may be transmitted at high power efficiency inherent to the class B or C power amplifier.
If kept as it is, however, a quantization noise pertaining to the PWM signal might be output as a noise out of a transmission band. To be specific, the PWM signal A′ (t) is what an error QA(t) corresponding to the quantization noise is added to the amplitude component A(t), and may be expressed in the formula 4 given below.A′(t)=A(t)+QA(t)  (Formula 4)
When the amplitude modulator 2 modulates the amplitude of the angle modulated wave having the fixed amplitude by use of the PWM signal A′(t) such as this, the quantization noise undergoing the amplitude modulation with a quantization noise Q(t) occurs also in the output of the amplitude modulator 2. A large proportion of the quantization noise becomes a component out of the transmission band and is therefore attenuated by a band pass filter (BPF) 4, thereby enabling a residual component QA′(t) of the quantization noise to be reduced.
Thus, in the transmission circuit apparatus (prior art) depicted in FIG. 1, the transmission signal with large fluctuations in envelope may be transmitted at the high efficiency with no distortion.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 2002-325109
The quantization noise occurs in the process of the modulation made by the Σ−Δ modulator described above. Therefore, in the case of desiring to vary average power of the transmission signal (such as decreasing the output power), if trying to realize power control by controlling an amplitude of the signal, it is hard to satisfy standards for the noise power out of the transmission band.